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The U.S. Air Force will try to launch its second secretive X-37B
space plane from Florida again today (March 5), after bad weather
foiled the first attempt yesterday.

The unmanned X-37B mini-shuttle — which is known as
Orbital Test Vehicle 2, or OTV-2 — is now slated to launch at
5:46 p.m. EST (2246 GMT) today from the Cape Canaveral Air Force
Station. The reusable space plane will ride to orbit atop an
Atlas 5 rocket to begin its mysterious mission, if the weather
allows.

Two launch attempts
were thwarted yesterday due to high winds and thick cumulus
clouds. The latter posed an unacceptable lightning risk,
according to launch provider United Launch Alliance (ULA), which
is overseeing launch preparations.

Launch crews also had to replace a faulty piece of equipment
needed for today's launch attempt, forcing Air Force officials to
skip a planned 4:09 p.m. EST (2109 GMT) launch attempt — one of
two flight opportunities for today, ULA officials said. [ Photos:
First Flight of the X-37B Space Plane ]

If the X-37B space plane does not launch today, another attempt
is possible on Monday (March 7). The Air Force would stand down
on Sunday to give launch crews time to rest, ULA officials said
in a statement today. If the lousy weather persists, more
significant delays could ensue.

ULA officials said the current launch window closes Monday to
make way for NASA's space shuttle Discovery. Discovery is slated
to land at nearby Kennedy Space Center on Wednesday (March 9),
closing out its final mission ahead of the shuttle program's
retirement later this year.

Second mystery mission for the X-37B

OTV-2's launch, whenever it takes place, will mark the start of
the X-37B program's second space mission. The Air Force's other
X-37B vehicle, known as OTV-1, returned to Earth in December 2010
after spending 224 days orbiting the Earth.

What OTV-1 was doing during its long space sojourn is not
entirely clear, because X-37B missions and payloads are
classified. Some observers have speculated that the vehicles may
be the vanguard of a new space
weapon fleet.

The Air Force has denied this rumor repeatedly, claiming that the
X-37B space plane's main mission is to test out new
technologies — such as elements of guidance, navigation and
control systems — that could be incorporated into future
satellites.

That's probably true, experts say. And the hardware in question
may be gear designed to go into next-generation U.S. spy
satellites — hence all the secrecy.

A robotic mini-shuttle

The unmanned, reusable X-37B spacecraft resembles a miniature
version of NASA's space shuttles. The spacecraft is about 29 feet
long and nearly 15 feet wide (8.8 by 4.5 meters), with a payload
bay about the size of a pickup truck bed. [ Infographic: The
X-37B Space Plane ]

The X-37B launches vertically inside the nose cone of a rocket
and lands horizontally on a runway, like a space shuttle. It has
a solar array that generates power, allowing the craft to remain
in space for up to nine months at a time, according to Air Force
officials.

The X-37B is built by Boeing's Space and Intelligence Systems
division for the Air Force. NASA originally used the space plane
as an experimental test bed until funding for the project ran out
in 2004.The vehicle then passed to the Defense Advanced Research
Projects Agency and was ultimately turned over to the Air Force
in 2006.

You can follow SPACE.com senior writer Mike Wall on
Twitter:@michaeldwall.